A survey has revealed that 14 per cent of valid third party SSL certificates have been issued using MD5 signatures, which have recently been demonstrated to be vulnerable to attack.
According to Netcraft's SSL Survey, the signatures have been demonstrated to be vulnerable to attack by producing a fake certificate authority, signed by a widely trusted third party authority.
Researchers were able to achieve this by producing a hash collision They submitted valid certificate requests to a certificate authority while producing a second certificate that had the same signature but entirely different details.
When the authority signed the valid certificate, the signature also applied to the invalid certificate, allowing the researchers to spoof any secure website of their choice.
The survey found that 135,000 valid third party certificates were using MD5 signatures on public websites, around 14 per cent of the total number of valid SSL certificates in use.
Of the 128,000 RapidSSL certificates in use on public sites, all were signed with MD5 while some smaller authorities continue to use MD5, while some use a small number of certificates from Thawte and VeriSign.
VeriSign has stated that it has stopped using MD5-signing for RapidSSL certificates, and will have phased out MD5-signing across all their certificate products by the end of January 2009.
Other affected authorities are likely to follow suit, as SHA1 is well established and is already in use for the majority of SSL certificate signing, so it should be simple to switch to using this more secure alternative. Once it is impossible to obtain new certificates signed with MD5, this attack will be neutralised.